Document DE 199 55 508 C1 describes a supercharged internal combustion engine with an exhaust-gas turbocharger which comprises in the exhaust section of the engine an exhaust-gas turbine with a variably adjustable turbine geometry, and in the induction section a compressor, which is driven via a shaft of the exhaust-gas turbine and by means of which induction combustion air is compressed to an elevated boost pressure. The active turbine inlet cross section in the exhaust-gas turbine can be variably controlled by means of the variable turbine geometry, thereby providing the option of providing for different flow conditions in the exhaust-gas turbine for different load and operating states of the internal combustion engine so as to achieve optimum power both in engine operating mode and in engine braking mode.
An additional passage is formed in the compressor of the exhaust-gas turbocharger, extending approximately parallel to the axial compressor intake passage, in which the compressor impeller is rotatably mounted. The additional passage opens out into the compressor intake passage from the radially outer side adjacent the compressor impeller, so that combustion air which is supplied via the additional passage impinges on the compressor impeller blades and applies a driving torque to the compressor impeller. As a result, it is possible for the compressor to be operated in turbine mode in certain operating states of the internal combustion engine—in particular at low engine load and low engine speed. With additional rotary energy then being supplied to the compressor impeller, fluctuations in the speed of the compressor can be reduced. However, this intervention into the limiting of the fluctuations of the rotational speed of the exhaust-gas turbocharger is restricted to the lower load and speed ranges of the internal combustion engine, when the induction pressure in the induction section downstream of the compressor is lower than the ambient pressure, resulting in the pressure differential required for the impeller to operate as a turbine.
Furthermore, the internal combustion engine which is disclosed in DE 199 55 508 C1 is provided with an exhaust-gas recirculation device, which comprises a recirculation line between the exhaust section upstream of the exhaust-gas turbine and the induction section downstream of the compressor. An adjustable blocking valve is arranged in the recirculation line, and this valve can be opened in operating states in which the exhaust-gas back pressure exceeds the induction pressure, so that a mass flow of exhaust gas can be passed into the induction section. This makes it possible to reduce the nitrogen oxides, in particular when the internal combustion engine is operating under partial load.
A further supercharged internal combustion engine with exhaust-gas recirculation is described in DE 198 33 134 C1. A gas store can be used to store exhaust gas from the exhaust section and feed it into the induction section upstream of the compressor on demand, so that in addition to reducing nitrogen oxide, it is also possible to provide additional driving energy for the compressor impeller, whereby the fluctuations in rotational speed of the exhaust-gas turbocharger can be reduced further. When the pressurized exhaust gas flows onto the compressor impeller in the axial direction, however, it should be borne in mind that the compressor impeller blades are designed to compress the mass flow supplied, with the result that kinetic energy of the compressor impeller is converted into potential energy of the gas volume downstream of the compressor. On the other hand, in turbine mode of the compressor, the kinetic energy of the incoming mass flow can only be converted into driving energy for the compressor impeller to an insufficient extent, since the compressor impeller blades are not optimized for turbine operation.
Document DE 42 13 047 A1 discloses a compressor for an internal combustion engine, which to broaden the compressor characteristic diagram has what is described as a characteristic diagram-stabilizing measure (CDSM), which shifts the pump limit in the compressor characteristic diagram toward lower mass flows. The CDSM is realized by providing a circulation chamber in the compressor intake passage, which circulation chamber extends coaxially with respect to the compressor intake passage but is separated from the latter by means of a contour ring, the circulation chamber being in communication with the compressor intake passage in the region of both its end sides. The circulation chamber, which extends axially beyond the rotor inlet plane of the compressor impeller, has the function of returning a partial mass flow that has been fed to the compressor impeller counter to the main flow direction and then reintroducing it into the main flow in the compressor intake passage. This produces an annular flow which effects the shift in the pump limit. To enable the annular flow to form, the contour ring is made relatively short in the axial direction and has relatively large flow openings in the region of its two end sides.
To create an enlarged buffer volume, the circulation chamber can be connected to various gas-containing components of the internal combustion engine, for example to the crankcase, the air filter or the exhaust-gas recirculation. In any event, it is an object of this additional connection to increase the volume of the circulation chamber. This additional buffer volume alleviates the effect of pressure surges.
It is the object of the present invention to improve the efficiency and operational reliability of supercharged internal combustion engines. It is intended in particular to achieve lower fluctuations in rotational speed in the exhaust-gas turbocharger of the internal combustion engine.